Gene Therapy for Chronic Lymphoid Leukemia
Introduction Chronic lymphoid leukemia (CLL) is a neoplastic disease of lymphoblastic stem cells characterized by the clonal proliferation of mature B-cells. The reduced diversity of B-cell antigens results in a dangerous susceptibility to infections. CLL is the most common form of leukemia in adults. CLL arises from a diverse set of somatic mutations; three are described below. CLL is also associated with p53 oncogene mutations, an extra copy of chromosome 12 (trisomy 12), and deletion mutations on the long arm of chromosome 13. The various clinical presentations of CLL reflect the diversity of these mutations. NOTCH1 The notch1 ''mutation occurs in the gene encoding for a ligand-activated transcription factor that regulates lymphocytic stem cell differentiation. SF3B1 The SF3B1 mutation occurs in a splicing factor that carries out the precise excision of introns. BIRC3 BIRC3 mutations are associated with the worst clinical outcomes in CLL patients. ''Birc3 ''encodes a specific baculoviral inhibitor of apoptosis (Zhang, Kipps 2013). Treatment Current treatment strategies for CLL involve chemotherapy (with DNA replication-inhibiting purine analogs) and radiation therapy. More aggresive forms of CLL require painful and risky bone marrow transplants. However, CLL frequently persists despite these treatments. These refractory leukemias are the subjects of new experimental treatments involving genetically engineered T-cells. All B-cells, both normal and cancerous, display a CD19 antigen on their surface membrane. This antigen is the target of T-cell therapy. Healthy T-cells are harvested from the blood of the patient (autologous) or a donor (allogenic). A lentiviral vector containing the gene an anti-CD19 antigen as well as genes for a CD137 costimulatory domain and a CD3-zeta intracellular signaling domain was inserted into the T-cells. Lentiviral vectors were used as they can reverse transcribe their RNA and insert the DNA into the genome of a non-replicating cell like a T-cell. The lentivirus vector is safe to use as the genes for viral replication have been removed. The CD137 domain encodes a transmembrane protein that stimulates the production of more anti-CD19 T-cells. The CD3-zeta domain is an important but not well understood molecule that activates intracellular signaling pathways after antigen receptor binding (Kalos 2011). These T-cells are then transfused in a low dose to the patient. The anti-CD19 receptor detects the CD19 antigen on B-cells and activates the T-cell's cytolytic response, resulting in the lysis of the B-cell. The CD137 costimulatory domain is capable of stimulating the production of more anti-CD19 T-cells, allowing long-term management of CLL. At the time of transfusion, the response is weak. But after three weeks, the T-cells are replicated to sufficient numbers and the tumors begin to degrade (Porter, Kalos 2011). Risks Chimeric antigen T-cell therapy does pose significant risks as the modified T-cells cannot differentiate between normal and neoplastic B-cells. As a result, patients often suffer from lymphopenia and hypogammaglobulinemia, but these can be managed with B-cell transfusions and carefully limiting exposure to exogenous antigens. Another risk posed by T-cell therapy is the potential for graft versus host disease, where mutations arise in the therapeutic T-cells, causing them to go rogue and attack more than just host B-cells. Because of this, the T-cells are carefully screened by qPCR and southern blotting for the successful transduction and purity of the lentiviral insert (Brentjens 2011). References Zhang S, Kipps TJ. 2013. The Pathogenesis of Chronic Lymphocytic Leukemia. ''Annu. Rev. Pathol. Mech. Dis. '' Kalos M et al. 2011. T cells with chimeric antigen receptors have potent antitumer effects and can establish memory in patients with advanced leukemia. ''Sci Transl Med. '' Porter DL, Kalos M, et al. 2011. Chimeric antigen modulated T cells in chronic leukocytic leukemia. ''NEJM. Brentjens RJ et al. 2011. Safety and persistence of adoptively transferred autologous CD19-targeted T cells in patients with relapsed or chemotherapy refractory B cell leukemias. ''Blood. ''